This invention relates to communication devices and more particularly to communication devices that receive multiple replicas, sent from different transmitters, of the same information.
Digital communication systems include time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM telecommunication standard and its enhancements like GSM/EDGE, and code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and wideband CDMA (WCDMA) telecommunication standards. Digital communication systems also include “blended” TDMA and CDMA systems, such as cellular radio telephone systems that comply with the universal mobile telecommunications system (UMTS) standard, which specifies a third generation (3G) mobile system being developed by the European Telecommunications Standards Institute (ETSI) within the International Telecommunication Union's (ITU's) IMT-2000 framework. The Third Generation Partnership Project (3GPP) promulgates the UMTS standard. This application focusses on WCDMA systems for economy of explanation, but it will be understood that the principles described in this application can be implemented in other digital communication systems.
WCDMA is based on direct-sequence spread-spectrum techniques, with pseudo-noise scrambling codes and orthogonal channelization codes separating base stations and physical channels (terminals or users), respectively, in the downlink (base-to-terminal) direction. User terminals communicate with the system through, for example, respective dedicated physical channels (DPCHs). WCDMA terminology is used here, but it will be appreciated that other systems have corresponding terminology. Scrambling and channelization codes and transmit power control are well known in the art.
FIG. 1 depicts a mobile radio cellular telecommunication system 10, which may be, for example, a CDMA or a WCDMA communication system. Radio network controllers (RNCS) 12, 14 control various radio network functions including for example radio access bearer setup, diversity handover, etc. More generally, each RNC directs mobile station (MS), or remote terminal, calls via the appropriate base station(s) (BSs), which communicate with each other through downlink (i.e., base-to-mobile or forward) and uplink (i.e., mobile-to-base or reverse) channels. RNC 12 is shown coupled to BSs 16, 18, 20, and RNC 14 is shown coupled to BSs 22, 24, 26. Each BS serves a geographical area that can be divided into one or more cell(s). BS 26 is shown as having five antenna sectors S1-S5, which can be said to make up the cell of the BS 26. The BSs are coupled to their corresponding RNCs by dedicated telephone lines, optical fiber links, microwave links, etc. Both RNCs 12, 14 are connected with external networks such as the public switched telephone network (PSTN), the Internet, etc. through one or more core network nodes like a mobile switching center (not shown) and/or a packet radio service node (not shown). In FIG. 1, MSs 28, 30 are shown communicating with plural base stations: MS 28 communicates with BSs 16, 18, 20, and MS 30 communicates with BSs 20, 22. A control link between RNCs 12, 14 permits diversity communications to/from MS 30 via BSs 20, 22.
At the UE, the modulated carrier signal (Layer 1) is processed to produce an estimate of the original information data stream intended for the receiver. The composite received baseband spread signal is commonly provided to a rake processor that includes a number of “fingers”, or de-spreaders, that are each assigned to respective ones of selected components, such as multipath echoes or streams from different base stations, in the received signal. Each finger combines a received component with the scrambling sequence and the appropriate channelization code so as to de-spread the received composite signal. The rake processor typically de-spreads both sent information data and pilot or training symbols that are included in the composite signal.
FIG. 2 is a block diagram of a receiver 200, such as a mobile terminal in a WCDMA communication system, that receives radio signals through an antenna 202 and down-converts and samples the received signals in a front-end receiver (Fe RX) 204. The output samples are fed from Fe RX 204 to a rake combiner and channel estimator 206 that de-spreads the pilot channel, estimates the impulse response of the radio channel, and de-spreads and combines received echoes of the received data and control symbols. An output of the combiner/estimator 206 is provided to a symbol detector 208 that produces information that is further processed as appropriate for the particular communication system. Rake combining and channel estimation are well known in the art.
In order to accommodate the increasing demand for higher data rates in wireless user equipment (UE), such as cellular telephones, combination cellular telephones-personal digital assistants, and wireless-enabled personal computers, a high-speed downlink shared channel (HS-DSCH) was introduced in WCDMA. The HS-DSCH has a spreading factor of sixteen and can use several channelization codes simultaneously, with modulation being either quadrature phase shift keying (QPSK) or 16-ary quadrature amplitude modulation (16QAM). Each transmission time interval (TTI) includes one transport block, and the length of a TTI is three slots. After encoding, interleaving, and rate matching, the bits to be transmitted are distributed over one or more channelization codes. This is described in “Multiplexing and channel coding (FDD)”, 3GPP Technical Specification (TS) 25.212 ver. 5.6.0 (September 2003), for example. Since the chip rate in a direct-sequence CDMA system is typically constant, a higher spreading factor generally corresponds to a lower information bit-rate.
A multimedia broadcast and multicast service (MBMS) for the frequency division duplex (FDD) aspect of the WCDMA system is being standardized by 3GPP. MBMS is described in 3GPP Technical Specification TS 23.246 ver. 6.2.0 Technical Specification Group Services and System Aspects; Multimedia Broadcast/Multicast Service (MBMS); Architecture and functional description (Release 6) (April 2003) and Technical Report TR 23.846 ver. 6.1.0 Technical Specification Group Services and System Aspects; Multimedia Broadcast/Multicast Service (MBMS); Architecture and functional description (Release 6) (December 2002), among other places.
MBMS is intended to offer high-speed and high-quality broadcast, or multicast, transmission to mobile stations (UEs). To enhance the quality and bit rate of the MBMS transmission, it has been agreed in 3GPP to use large interleaving depths, i.e., large TTIs, to obtain interleaving gain, and to use multicast on Layer 1, i.e., the UE should be able to receive multiple replicas of the same bitstream from different base stations, each of which is a Node B in 3GPP vocabulary. It has also been agreed that UEs must be able to receive 128-kilobits-per-second (kbps) and 256-kbps bitstreams. The combination of long TTI, multicast on Layer 1, and bit rates as high as 128 kbps and 256 kbps results in very large buffer memory sizes for Layer 1 processing in the UE.
Further, it has been agreed that the UE should be able to do selective combining on the radio link control (RLC) level (Layer 2). This means that different bitstreams are separately processed on Layer 1, and the RLC entity selects transport blocks, called RLC protocol data units (PDUs), from the different streams based on whether they pass a cyclic redundancy check (CRC) or not. Instead of using RLC PDUs, selective combining can also be carried out between multiple antennas, polarization angles, etc. This kind of separate processing of bitstreams on Layer 1 also results in very large buffer sizes since all received bitstreams must be separately buffered. Selective combining is well known in the art.
Besides selective combining, another well-known combining technique is soft combining or maximum ratio combining. Soft combining, which UEs already perform on Layer 1, can save both buffer space and processing time since the bitstreams are combined early in the receiver chain and since the soft-combined bitstream is processed as one stream, not several. Soft combining is well known in the art, and is used by some kinds of turbo decoders and by hybrid automatic repeat request (HARQ) in high-speed downlink packet access (HSDPA) in 3G radio communication systems.
Generally, the buffer size required for soft combining increases linearly with the timing difference between the transmission links involved in a broadcast or multicast. On the other hand, the Layer 1 buffer size required for selective combining is substantially constant with timing differences. This and other differences between selective and soft combining have caused problems for UE designers, who have not been able to optimize their designs for both types of combining simultaneously.